This application claims the priority of Korean Patent Application No. 2002-75299, filed on Nov. 29, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method and apparatus for quickly removing false contours in a pulse number modulation (PNM) digital display device including a plasma display panel (PDP).
2. Description of the Related Art
In conjunction with the development of HDTV broadcasting, ultra-thin display devices, such as plasma display panels (PDPs) or digital micromirror devices (DMDs), have become more popular. Such matrix-type display panels can represent gray scales using a pulse number modulation (PNM) technique.
More specifically, in order to represent a moving picture on the screen of a display device, more than 24 picture frames should be consecutively displayed per each second. In addition, in order to represent a 24 bit-color picture (R: 8 bits×G: 8 bits×B: 8 bits), the amount of illumination should be regulated according to 256 (8-bit) gray scales. PDPs generally adopt a PNM technique where one field is divided into a plurality of subfields and the illumination of each of the subfields is controlled using a number of sustain pulses.
This type of PDP driving method works very well when dealing with still pictures. However, when encountering an occasion when a viewer's view point moves, the above PDP driving method may result in distortions of a picture displayed by a display device. This phenomenon is called false contours (of a moving picture). False contours could be generated depending on the extent of asynchronism between illumination and a result of multiplying the illumination time of pixels by the migration speed of the viewer's view point.
In other words, in a PNM-based method of representing gray scale, a subfield illumination point temporally varies in accordance with the variation of input gray scale. Therefore, the gray scale of a moving picture may have false contours that really do not exist on an original picture because of temporal variations of the subfield illumination point, while the gray scale of a still picture can be well represented without any distortions. In other words, the temporal variation of illumination is represented as a spatial variation, which results in false contours.
There are a few related art methods for addressing the problem of false contours, e.g., a method of selecting a combination of subfields for minimizing the variation of a subfield illumination pattern having a largest. illumination weight value, a method of inserting equalizing pulses into a predetermined area where false contours are anticipated to occur, and a method of scattering false contours.
In the method of selecting a combination of subfields, which has been disclosed in U.S. Pat. Nos. 6,268,890B1 and 6,310,588B1, false contours are prevented from occurring by arranging subfield illumination weight values in a monotonously increasing or decreasing manner and selecting a combination of the subfield illumination weight values that can prevent a subfield having a greatest illumination weight value from being turned on as much as possible. Even though it can prevent false contours from occurring to some extent by diffusing the variation of a subfield illumination distribution in a temporal direction, this method cannot completely prevent the variation of the subfield illumination distribution and thus, cannot effectively get rid of false contours. In addition, when the amount of motion is large, resulting in a large error, noise can be too easily detected through the error diffusion.
In the method of inserting equalizing pulses into a predetermined area where false contours are anticipated to have occurred, which is taught by Toda in U.S. Pat. No. 6,097,368, false contours can be prevented from occurring by anticipating when a transition among subfields where false contours possibly occur will be generated inserting equalizing pulses before the moment of time when the transition is anticipated to occur. However, this method needs a sophisticated motion estimator to obtain precise equalizing pulses, which is a major setback to the implementation of this method. In order to prevent false contours in this method without using the motion estimator, there has been suggested a predetermined method in which a plurality of optimal equalizing pulse codes for a current gray scale are calculated off-line and stored in ROM and an optimal equalizing pulse code that results in as less false contours as possible, but there is still a clear limit in effectively removing false contours.
In the method of scattering false contours, which is taught by Watanabe in U.S. Pat. No. 6,088,012, false contours can be reduced by dividing a subfield having a heavier illumination weight into smaller subfields and placing the smaller subfields at positions before and after the subfield having a heavier illumination weight. However, this method may cause blurry moving pictures because it uses temporally distant, upper weights to represent a high brightness gray scale.